How to Make It
In a large glass baking dish, combine 1/4 cup of the oil with the port, vinegar, lemon juice, mustard, rosemary, peppercorns, garlic and shallots. Add the lamb and turn to coat. Let stand at room temperature for 40 minutes.
Meanwhile, light a grill or preheat the oven to 375°. Drain the lamb and scrape off the marinade. Rub the racks with the remaining 1 tablespoon of olive oil and season with kosher salt and pepper. Grill over a medium-hot fire for 10 to 15 minutes, turning often, until an instant-read thermometer registers 130° for medium rare. Alternatively, heat a large, ovenproof skillet and sear the lamb over moderately high heat until browned on both sides, 5 to 6 minutes total. Transfer the skillet to the oven and roast the lamb for 12 minutes for medium rare.
Transfer the lamb to a cutting board and let rest for 5 minutes. Cut the lamb between the bones into chops, arrange 4 chops on each plate and serve.